1. Field of the Invention
This invention relates to materials of construction for a solar central receiver and more particularly to the use of low cycle fatigue 625 alloy as the material used to fabricate a solar central receiver containing high temperature sodium-potassium nitrate salt as the heat transport fluid.
2. Description of the Related Art
Presently, materials used to contain 565.degree. C. sodium-potassium nitrate salt in solar central receiver applications include 304 and 316 austenitic stainless steels and Incoloy 800 nickel-iron-chromium alloy. This class of materials possesses: high coefficients of thermal expansion, low yield and creep strengths, low thermal conductivities, low thermal fatigue properties and are susceptible to chloride stress corrosion cracking. However, they do have; excellent salt corrosion resistant properties up to 600.degree. C., excellent weldability and fabricability, and are acceptable to the ASME Boiler and Pressure Vessel Code.
When used in a molten nitrate salt, solar central receiver the construction material for the solar absorption panel tubes should be: resistant to the molten salt's strong oxidation properties, resistant to chloride stress corrosion cracking, economically fabricated, weldable, acceptable to the ASME Boiler and Pressure Vessel Code and table to withstand the severe thermal strains caused by the through wall and across diameter temperature gradients. These strains, which are directly proportional to the material's thermal expansion coefficient, set the receiver's size by restricting the absorbed solar flux to a value determined by the material's allowable fatigue strain level for the imposed number of daily sun and cloud cover cycles over the receivers lifetime.
Because a receiver's radiation and convection thermal losses are directly proportional to its area and temperature distribution, a smaller receiver with equivalent temperatures has lower thermal losses. However, because the light reflected by the solar plant's sun collection field (heliostats) onto the receiver more easily misses a smaller receiver its light spillage losses are greater. Still, the reduction in thermal losses off-sets the light spillage losses except for very small receivers. Also, smaller receivers designed to achieve their reduced size by improved material properties, such as; a lower coefficient of thermal expansion combined with a greater thermal fatigue strength have lower fluid flow pressure losses and reduced capital costs because they have fewer shorter tubes of a diameter like the lesser material's larger size receiver.